Natural gas and other gases, such as combustible gases, are routinely transported to users via high pressure pipelines. Conventionally, natural gas extracted from a production well in one part of the country is first locally conditioned and then supplied to a high pressure gas transmission pipeline for long distance transfer to users sometimes thousands of miles away. Because of the frictional losses in the pipeline, compressor stations are regularly located along the length of the pipeline to maintain the high pressure therein.
Near a user location, it is conventional to provide a metering and regulating station whose function is to reduce the gas pressure to a level consistent with local transmission to one or more users such as industrial customers, electrical utilities, domestic customers, etc. A typical metering and regulating station would include a pressure regulating system including one or more pressure regulators or regulating valves each of which includes a regulated flow control opening established by a variable orifice or movable diaphragm that throttles the gas for reducing its pressure to a level compatible with local distribution and/or use. Conventional pressure regulators or regulating flow control valves are usually pneumatically operated by a downstream sensor so as to maintain a preselected range of downstream pressures in the face of changes in local demand, etc., and also usually in face of changes in upstream pressure.
Throttling of a high pressure gas wastes the energy expended in the process of reducing the gas pressure; and one solution to recovering some of this energy is to expand the gas in an expander. In such case, the high pressure gas expands in a rotary machine such as a radial flow expander coupled to a generator, and the pressure reduction in the expander is converted to electricity. Thus, the expander/generator constitutes an energy recovery, pressure reducer that duplicates the function of a conventional pressure regulating valve in a pressure reducing station. In this manner, some of the energy expended to pressurize the gas is recovered.
Because a temperature drop accompanies the pressure drop through the expander, any moisture in the gas is likely to freeze detrimentally affecting the operation of the expander and utilization devices downstream of the expander. It has been suggested therefore to preheat the gas before it is applied to the expander, and to this end, it is conventional to burn fuel for this purpose.
This solution to energy recovery, while conventionally used in refineries, is not presently acceptable in pipeline systems because the expanders and their associated equipment adversely impact on the operational of the pipeline. Where an energy recovery, pressure reducer is used in a pressure reducing station on a pipeline, conventional design requires the energy recovery, pressure reducer to shunt the conventional pressure regulator or regulating valve of the metering and regulating station to permit alternative operation when one or the other of the components must be taken off-line for maintenance, for example. Switching operation from the energy recovery, pressure reducer back to the pressure regulator or regulating valve is a major problem in a pipeline because of shock waves introduced into the pipeline by sudden changes in flow rate accompanying such switching. These shock waves travel upstream and downstream of the pressure reducing station and adversely affect upstream pressure reduction stations as well as upstream compressor stations and other components. Furthermore, the down time associated with component failures in a energy reducing, pressure reducer is a further problem.
It is therefore an object of the present invention to provide a new and improved energy recovery, pressure reducing system and/or apparatus and a method for using the same which eliminates, or substantially reduces, the above-mentioned problems with the prior art.